1. Field of the Invention
The invention relates to a method for determining a trellis describing a plurality of paths starting from an initial state and having a predefined length, said trellis being used for a transmission between a transmitter and a receiver.
The invention also relates to a modem using such a method, a communication system comprising such a modem, and a computer program comprising code means for executing the steps of said method.
The invention is useful in all cases where the paths through the trellis are short and the number of states of the trellis is small.
For example, it may be applied to V90 spectral shaping. V90 is a recommendation of the International Telecommunications Union (ITU) dealing with a digital modem and an analog modem pair for use on the public switched telephone network.
2. Description of the Related Art
It is known, from paragraph 5.4.5.5. of the V90 Recommendation, to use a trellis in a digital modem to implement a coding algorithm called xe2x80x9cspectral shapingxe2x80x9d algorithm. According to this recommendation, the length of the trellis is an integer lying between 0 and 3, selected by the analog modem during training procedures.
The problem is that the length of this trellis may change at each connection. Thus, at each connection, the new trellis needs be determined, which involves a lot of calculations.
One of the objects of the invention is to provide a simple method of determining the trellis for a transmission between a transmitter and a receiver.
According to the invention, all possible paths through the trellis are described branch-by-branch for the maximum length of the trellis. Then, all these paths are stored, for example, in a matrix called Reference Matrix. Paths to be used for a smaller length of the trellis are deducted from said Reference Matrix, from the length of the current trellis, from the maximum length, and from the initial state of the paths.
The invention is advantageous because it avoids the storage of all the possible configurations of paths, and thus it saves memory space.